It may seem counter-intuitive to argue that shorter maternity leave would improve things for Britain's mothers. But in our latest briefing about the government's proposed changes to the UK's parenting leave system we set out what we at the Fatherhood Institute think are persuasive arguments for setting maternity leave at 18 weeks – which is what the government originally proposed in its modern workplaces consultation.

If maternity leave were set at 18 weeks, mothers would still retain the right to the full 52 weeks leave they have now. The difference is that after 18 weeks it would be parental leave – which can be flexible (taken in blocks of different lengths, and part-time or full-time) and shareable with their partner.

A more flexible approach to parenting leave is needed, allowing fathers and mothers to share parenting responsibility and balance work and family commitments. That's in tune with what parents want.

Last month's Queen's speech confirmed that the coalition plans to move to a more flexible system. The question now is how much of the total parenting leave on offer fathers should be permitted to share – and how much should be reserved (restricted) for the mother's use only as maternity leave.

The charity Working Families favours the retention of 26 weeks' maternity leave. But is this really the best deal for mothers?

Countries with a good track record on gender equality tend to have very short periods of maternity leave: for example, just two weeks in both Sweden and Iceland, and six weeks in Norway. Of course this is not the only leave available to mothers: all mothers can take much more – but as flexible parental leave, not as one continuous block of maternity leave.

Equality for women requires fathers play an equal part in child-rearing. When less leave is publicly "earmarked" for mothers, the shift in cultural perceptions should mean that men and women are considered equal in terms of the ability to care for their child, both in fact and in law. This should increase caring by fathers and help women achieve greater parity with men in the workplace. Evidence from Sweden suggests that a mother's future earnings increase on average by 7% over a four-year period for every month of leave the father takes. Iceland reserves 13 weeks of its parental leave for mothers, 13 weeks for fathers, and designates an additional 13 weeks as joint leave to be used as the family wishes. Ninety-one percent of Icelandic fathers now take leave, with 83% using all their own reserved leave and 23% also taking some of the joint leave.

In the UK, if maternity leave were to be set at 18 weeks rather than 26 weeks, mothers could start flexibility earlier. And they would have a longer period in which to restart leave if their needs changed: if a woman returns to work and breaks her maternity leave – as many now do, since the later weeks are paid only at a low, statutory pay rate (as is the current two weeks' paternity leave) – there is no way to restart it. Under the new system, she would be able to restart leave after going back to work, so long as she gave her employer sufficient notice.

Designating a clear majority of the first year's leave as parental leave, not maternity leave, would send out a strong signal that fathers are to be equal partners in parenting. And it would give mothers and fathers the ability to think more creatively about how they might share the responsibility for hands-on caring, while retaining women's ability to take up to a year off if that seems right for their family.